The present invention relates to holographic data storage media.
Many different types of data storage media have been developed to store information. Traditional media, for instance, include magnetic media, optical media, and mechanical media to name a few. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
In traditional media, individual bits are stored as distinct mechanical, optical, or magnetic changes on the surface of the media. For this reason, medium surface area may pose physical limits on data densities.
Holographic data storage media can offer higher storage densities than traditional media. In a holographic medium, data can be stored throughout the volume of the medium rather than the medium surface. Moreover, data can be superimposed within the same media volume through a process called shift multiplexing. For these reasons, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
In holographic data storage media, entire pages of information can be stored as optical interference patterns within a photosensitive optical material. This can be done by intersecting two coherent laser beams within the optical material. The first laser beam, called the object beam, contains the information to be stored; and the second, called the reference beam, interferes with the object beam to create an interference pattern that can be stored in the optical material as a hologram. When the stored hologram is later illuminated with only the reference beam, some of the reference beam light is diffracted by the hologram. Moreover, the diffracted light creates a reconstruction of the original object beam. Thus, by illuminating a recorded hologram with the reference beam, the data encoded in the object beam can be recreated and detected by a data detector such as a camera.
The invention is directed towards holographic data storage media, holographic data storage systems, and methods for making holographic data storage media. The holographic data storage media may incorporate thermoplastic substrates having reduced substrate thicknesses. Moreover, in some embodiments, holographic data storage media incorporate thermoplastic substrates within a particular thickness range.
In one embodiment, a holographic data storage medium may include a first thermoplastic substrate portion having a thickness less than or approximately equal to 2 millimeters, a second thermoplastic substrate portion having a thickness less than or approximately equal to 2 millimeters, and a holographic recording material sandwiched between the first and second thermoplastic substrate portions. By way of example, the first and second thermoplastic substrate portions may be made of at least one of the following: polycarbonate, polymethylmethacrylate (PMMA), and amorphous polyolefin. The holographic recording material may be made of a photopolymer.
The holographic data storage medium, for instance, may take the form of a disk or a card. The first and second thermoplastic substrate portions may be injection molded substrate portions. As will be described in detail below, an edge wedge phenomenon associated with injection molded thermoplastic substrates can make fabrication of holographic data storage media challenging. To overcome problems introduced by the edge wedge phenomenon, the invention may involve the use of substrate portions with reduced thicknesses. For instance, each of the first and second thermoplastic substrate portions may have thicknesses less than or equal to approximately 2 millimeters, less than or equal to approximately 1.2 millimeters, or even less than or equal to approximately 0.6 millimeters.
Optimal substrate thicknesses may have a lower limit determined by other variables such as birefringence and stiffness. Therefore, in one particular embodiment, each of the first and second thermoplastic substrate portions have thicknesses less than 1.3 millimeters and greater than 0.5 millimeters. 1.3 millimeters to 0.5 millimeters, for instance, may define an optimal thermoplastic substrate thickness range.
In other embodiments, the invention may comprise a holographic data storage system. The system may include a laser that produces at least one laser beam and optical elements through which the laser beam passes. The system may also include a data encoder, such as a spatial light modulator, that encodes data in at least part of the laser beam. In addition, the system may include a holographic recording medium that stores at least one hologram. The holographic recording medium, for instance, may include one or more of the features mentioned above, such as thin thermoplastic substrate portions. The system may also include a data detector, such as a camera, that detects the hologram.
In yet another embodiment, the invention may comprise a method of fabricating holographic media. The method may include injection molding a first substrate portion and a second substrate portion, and depositing a photopolymer between the first and second substrate portions. Injection molding the first and second thermoplastic substrate portions, for instance, may comprise injection molding the first and second thermoplastic substrate portions to have sufficiently thin substrate thicknesses. Depositing the photopolymer may comprise injecting the photopolymer between the first and second substrate portions. For instance, for a disk shaped medium, the photopolymer may be injected by center dispensing the photopolymer through an inner diameter of the substrate portions of the medium. The method may also include forcing the first substrate portion onto an upper reference plane and forcing the second substrate portion onto a lower reference plane. The photopolymer may then be cured in situ.
Substrate thicknesses less than or equal to approximately 2.0 millimeters, less than or equal to approximately 1.2 millimeters, or less than or equal to approximately 0.6 millimeters may be highly advantageous. In particular, substrate thicknesses in these ranges may minimize the negative effects of the edge wedge phenomenon that is described in detail below. Briefly, the edge wedge phenomenon is the result of differential cooling of the thermoplastic material as it solidifies in an injection molding cavity. The differential cooling, for instance, can result in substrates that exhibit cusps at the substrate edges that are thicker than the average thickness of the substrate.
Other factors, such as birefringence and stiffness, however, may make thicker substrates more desirable. The range of 0.5 millimeters to 1.3 millimeters, for instance, may define an optimal thermoplastic substrate thickness range for sandwich construction holographic media.
Additional details of these and other embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages will become apparent from the description and drawings, and from the claims.